A porous resin film made of a synthetic resin has been used as a filtration material for water purifiers, air purifiers and the like, synthetic paper, sanitary materials, medical materials, construction materials, air permeation sheets for agriculture, various kinds of separators for batteries, separators for electrolytic capacitors and the like and further it has been desired to improve and develop such a resin film for making the film thin while maintaining the strength of the porous resin film in all of these applications.
Particularly, in recent years, since a lithium secondary battery, which has been used in mobile appliances such as cellular phones and notebook type personal computers, has a high energy density for the volume and the weight as compared with nickel-hydrogen secondary battery or the like, production and consumption of the lithium secondary battery have been increasingly expanded since it was commercially made available in the beginning of 1990s.
Along with further improvements in the properties and capabilities of various kinds of the mobile appliances, the lithium secondary battery employed as a main power source has also been required to have improved properties and capabilities and similarly to both negative and positive electrodes, the physical properties of a separator made of a porous resin film are also required to give high capacity durability, safety, and the like.
A battery separator is required to be thinner and have a high porosity and high ventilation property, in addition to the electrically insulating property for both electrodes, which is an intrinsic property of the separator, since as the ion permeation is higher, the inner resistance is decreased and the properties of the battery are improved.
However, the insulating property, which is a function of the separator, and decrease of the inner resistance are contradictory and therefore they cannot be satisfied simply by making the separator thin, and it is required to give good size stability, wettability to an electrolytic solution, retention property, and corrosion resistance and to pay attention to chemical and electric insulation stability, excellencies of mechanical properties such as strength against penetration, workability at the time of rolling, cost and the like.
Additionally, in terms of safety, it is also very important that a separator for lithium secondary batteries is provided with a shut down function for stopping the battery reaction by increasing the temperature of the inside of the battery and thereby melting the resin and shutting its pores when an abnormal current is generated due to erroneous connection or the like.
For the above-mentioned requirements, presently, the thickness and the like of selectable resins and films have been limited for every application.
So far, porous films used as materials of sanitary products such as diapers and bed covers and clothing such as gloves have been used as separators for lithium secondary batteries.
However, researches and investigations of porous films more suitable to satisfy the above-mentioned requirements have been performed and a method was proposed for obtaining a porous film by mixing resin particles with an average particle diameter of 0.01 to 10 μm and a β-nucleating agent to polypropylene for obtaining a polypropylene composition, producing a film from the polypropylene composition, and stretching the film by rolls (refer to Patent Document 1).
Further, paying attention to stretching unevenness and dispersion of the pore size of a porous film in relation to the inner resistance of a battery, a method was proposed for obtaining a porous film with high evenness by adding as a filler a resin with high melt viscosity and low melt elongation to a thermoplastic resin, melting and kneading the mixed resin, and stretching the kneaded resin (refer to Patent Document 2).
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 9-176352
Patent Document 2: JP-A No. 2002-264208